The johns knew her as “Angela,” a 16-year-old whom pimps sold on the website Backpage.com, updating the ads throughout the day to ensure that a picture of her lingerie-clad body would remain at the top of the advertising page’s daily offerings.

The pimps in the case have received sentences ranging from probation and short jail stays to four years.

Angela, now 20 and attending Metropolitan State University of Denver, believes action is needed to scrub the Internet of sites where sex with children is purchased.

“I think there should be some consequences if they have something on the Internet that they know is a direct link to child sex trafficking. I am living proof that this is going on,” she said. “Most of the time, girls don’t end up going to college; the kids normally wind up dead or go back to (prostitution).”

The problem is more complex than those who condemn the company believe, and Backpage is taking measures to keep traffickers off the site, said Liz McDougall, Backpage general counsel.

“Identifying and vilifying a single U.S. website as the cause of the problem and the key to the solution are ill-founded and unproductive,” she wrote in an e-mail.

Federal courts have ruled that parts of the 1996 Communications Decency Act provide immunity from state prosecution to classified-ad sites, saying they can’t be held responsible for the actions of third-party advertisers, said Colorado Attorney General John Suthers.

Last July, the National Association of Attorneys General wrote to Congress seeking an amendment to the act, arguing that the law needs to be updated so local prosecutors can take action against those who use technology to promote sexual exploitation of children.

Congress didn’t act on the request.

“We still see Backpage as a very irresponsible corporate actor,” Suthers said.

Backpage users can find roommates, child care, classes and workshops, and can buy or sell anything from furniture to vehicles. The site’s adult-services section is home to ads for escort services, strip clubs and massage parlors.

The FBI Rocky Mountain Innocence Lost Task Force recovered 140 minors from pimps throughout the Denver metro area between Jan. 1, 2012, and March 15, 2014, according to Denver police Sgt. Daniel Steele, head of the task force.

During the same period, the task force arrested 40 people for pimping and other trafficking-related offenses, Steele said. Most were using the website to sell minors, he said.

Steele gives Backpage high marks for cooperating with law enforcement in trafficking cases.

However, the company allows customers to use prepaid debit cards, which can be impossible to trace if the ad is found to be for an illegal transaction, and Backpage doesn’t verify the identity of those buying ads, Steele said.

“If they were just doing a better job of making sure that it is a legitimate person posting the ads,” he said, “they would be hard to argue with.”

Backpage has about 80 staff members who operate a prevention system that includes an automated filter and two levels of human review to catch ads that indicate illegal activity, particularly sexual exploitation of a child or prostitution, McDougall said.

The system includes more than 39,000 items that trigger a filter, including such phrases as “barely legal,” “trip to the islands,” “girlfriend experience” and variations.

Every ad submitted to the adult-services section is also reviewed by a trained moderator who checks the text and all images for possible illegal content, McDougall said.

The company’s efforts fall short of what is needed, said Staca Shehan, director of the NCMEC case-analysis division. Among other things, Backpage could take steps to verify that the person advertised is over 18 and that the subject of the ad is the one buying it, Shehan said.

“I also think they could do a lot with analyzing their own internal data,” Shehan said.

For instance, technology could be used to spot phone numbers that show up repeatedly but are linked to ads for a number of people.

Shehan doesn’t dismiss concerns that shutting down adult-services advertising could drive the business offshore and increase the difficulty of policing child sex trafficking.

“However,” she said, “you can’t allow a crime to continue because of concerns you might have if you change the current system.”

A general assignment reporter for The Denver Post, Tom McGhee has covered business, police, courts, higher education and breaking news. He came to The Post from Albuquerque, N.M., where he worked for a year and a half covering utilities. He began his journalism career in New York City, worked for a pair of community weeklies that covered the west side of Manhattan from 14th Street to 125th Street.

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